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www.elsevier.com/locate/sedgeo
Sedimentary Geology 163 (2004) 211–228
Late-Pleistocene seismites from Lake Issyk-Kul,
the Tien Shan range, Kyrghyzstan
Dan Bowmana,*, Andrey Korjenkovb, Naomi Poratc
aDepartment of Geography, Ben-Gurion University of the Negev, P.O. Box 653, BeerSheva 84105, Israelb Institute of Geosciences, Potsdam University, Postfach 60 15 53, D-14415 Potsdam, Germany
cGeological Survey of Israel, 30 Malkhe Yisrael Street, Jerusalem 95501, Israel
Received 26 March 2002; received in revised form 4 March 2003; accepted 4 June 2003
Abstract
The aim of the study is to record the occurrence of sediment deformation structures in one of the tectonically most active
areas on the globe, the Tien Shan range in Central Asia and to examine the significance of the deformations as indicators of
palaeoseismicity.
Soft-sediment deformation structures in form of balls and pseudo-nodules are exposed in the Issyk-Kul basin, within
interfingering beds of shallow lacustrine, beach and fluviatile origin. Additional deformation structures that were encountered
are: a complex and chaotic folded structure, giant balls and a ‘‘pillar’’ structure which has not been previously reported, where
marl intrudes down into coarse pebbley sand and forms pillar morphology. Liquefaction features and bedforms related to storm
and breaking waves were not encountered. Neither was there evidence of turbidites. Seven field criteria for relating soft-
sediment deformation to palaeoseismic triggering provide strong evidence for a seismic origin of the deformation structures.
Empirical relationships between magnitude and the maximum distance from an epicenter to liquefaction sites make the active
epicentral zone north of Lake Issyk-Kul, with its frequent high magnitude events, the most favorable source for the deformation
structures. Luminescence dating of the sediments gives a time window of 26F 2.1 to 10.5F 0.7 ka BP, indicating latest
Pleistocene seismic activity.
D 2003 Elsevier B.V. All rights reserved.
Keywords: Neotectonics; Seismites; Palaeoseismicity; Soft-sediment deformation; Tien Shan; Kyrghyzstan
1. Introduction
Soft-sediment deformation structures are common
in unconsolidated, loosely packed and saturated sands
interbedded with silt and some clay. They have been
recorded in many studies from all sedimentary envi-
0037-0738/$ - see front matter D 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0037-0738(03)00194-5
* Corresponding author. Fax: +972-8-647-2821.
E-mail address: [email protected] (D. Bowman).
ronments, in particular, from lacustrine beds (Hemp-
ton and Dewey, 1983; Tinsley et al., 1985; Anand
and Jain,1987; Scott and Price, 1988; Calgue et al.,
1992; Rodriguez-Pascua et al., 2000; Galli, 2000).
The soft sediments were described as having lost
strength through becoming semiliquid (Lowe, 1975).
Deformation of liquidized sediments without appli-
cation of much external force has been associated, by
Dzulynski (1966), with inverse density gradients
acquired at deposition, or during resedimentation into
Fig. 1. Location (a, b) and structural setting (c) of the northern Tien Shan belt, the Issyk-Kul lake and the basin.
D. Bowman et al. / Sedimentary Geology 163 (2004) 211–228212
Fig. 1 (continued).
D. Bowman et al. / Sedimentary Geology 163 (2004) 211–228 213
a tighter packing. In many cases, the deformations
were attributed to shaking by earthquakes. Soft-sed-
iment deformations of a plastic nature may, however,
also be triggered aseismically by rapid deposition and
unequal loading, by cyclic oscillation of storm
surges, by the force of downslope-driven density
currents or following a significant change in artesian
pressure.
The Tien Shan range (Fig. 1) is one of the most
seismically active regions of the world and is known
for major earthquakes (Dzhanuzakov et al., 1980;
Kondorskaya and Shebalin, 1982). Several lakes oc-
cupy depressions within this active range. The Middle
to Upper Pleistocene and Holocene lacustrine deposits
were susceptible to intense earthquake activity but no
previous attempts have examined their sediment de-
formation structures as indicators of palaeoseismicity
so as to extend backwards the record of active
seismicity.
The aim of the study is to locate, characterize and
date sediment deformation structures in the Issyk-Kul
Lake area, northern Tien Shan, and assess their
significance as indicators of palaeoseismicity, bearing
in mind the difficulties in distinguishing between
seismic and nonseismic triggering.
2. Study area
The Issyk-Kul lake area (Fig. 1) is a tectonic
ramp depression bordered by convergent thrust
faults, dipping in opposite directions (Chedia,
1993). In the north, the Issyk-Kul depression is
bounded by the Kungey ridge and by a set of en-
echelon thrust faults, i.e., the west Toguz-Bulak, the
Kultor and the northern Aksu and Taldy-Bulak
faults. The Terskey ridge bounds the depression in
the south along with the southern pre-Terskey fault
zone. The Miocene and Pliocene mark an era of
intensive orogenic uplift shown by the coarsening
upward of the 4000-m thick, sandy gravelly Issyk-
Kul formation (Fortuna, 1993). The Quaternary
deposits include lacustrine clays to giant glacier
boulders (Korjenkov, 2000). Maximum thickness of
the Cenozoic deposits in the Issyk-Kul depression is
5000 m.
D. Bowman et al. / Sedimentary Geology 163 (2004) 211–228214
The Issyk-Kul intermontane basin has been occu-
pied by lakes since Early Neogene (Voskresenskaya,
1983). The present lake has existed since Mid-
Pleistocene, about 700,000 years ago. Its maximum
level was 1675–80 m (Trofimov, 1990). The highest
possible lake level, before spilling over through the
Boom Gorge to the northwest, towards the Chu
valley, is today 1620 m (Fig. 1). During the Holo-
cene, the water level of Lake Issyk-Kul dropped to
110 m below the present level (Fig. 2), as indicated
by underwater shore terraces, submerged canyons, a
network of river channels and submerged human
settlements (Bondarev, 1983). Subsequently, in the
first half of the 19th century, the lake level rose to
1622 m. Since then the lake level has gradually
dropped towards its current 1606-m level. The fluc-
tuations of the lake level are related to climatic
changes superimposed upon tectonic movements
(Grigina and Fortuna, 1981).
The maximum length and width of Lake Issyk-Kul
are 179 and 60 km, respectively. The total shoreline
length is 662 km and maximum depth is 668 m. A
strong gradient of precipitation exists from the east
with 720 mm/year to the west with 120 mm/year.
Evaporation amounts to 836 mm per annum (Kri-
voshey and Gronskaya, 1986). The water is brackish:
salinity is 5.9 g/l (Romanovsky, 1990).
The receding lake level has cut a beach cliff at an
altitudinal range of 1620–1640 m. The cliff is com-
posed of interfingering alluvial and lacustrine sedi-
Fig. 2. The Issyk-Kul lake level fluctuations from Mid-Pleistocene on and
period 1860–1910 is based on reports of various researches and on cartog
period 1975–2000 had been lineary reconstructed to the recent 1606-m l
ments. An array of gravelly sandy beach bars extends
from the base of the cliff down to the recent shoreline,
reflecting the last stage of lake-level fall.
3. Methods
3.1. Field work
Extensive surveys were carried out along the
shores of the lake and the beach cliffs in order to
locate deformation structures. Detailed mapping was
undertaken at five locations (Fig. 3): along sections by
the Akterek outlet (stations 11, 15); by the outlet of
the Irdyk (station 18); at the Karakol river outlet
(station 17) and by the Choktal beach (station 10).
The altitudes of all the sections were tied by
leveling to the current lake level. At each station, a
systematic description of the stratigraphic column was
done. The following sedimentary characteristics were
recorded in detail: texture, including grain size; round-
ness and sorting; thickness and regularity of the
bedding; lenticular features; cyclic bedding; cross-
bedding; microstructures and micro-cross-lamination.
The deformation structures were measured in terms of
their size and geometric characteristics including
thickness and length, symmetry, shape, degree of
penetration and isolation, top and bottom contacts,
structural gradient, composition of the host unit and
lateral continuity.
for the period 1860–2000 in detail, following Trofimov (1975). The
raphic sources. From A onward, there was regular monitoring. The
ake level.
Fig. 3. The main sections studied along the Issyk-Kul shoreline. Soft sediment deformation, dates and the bases of the largest deformation units are indicated. Only sections 15, 17 and
18 are located altimetrically. Only well-developed load casts are shown.
D.Bowmanet
al./Sedimentary
Geology163(2004)211–228
215
Table 1
Luminescence dating–Field and laboratory results from the Issyk-Kul samples
Sample Depth
(m)
De (Gy) K KF
(%)
K
(%)
U
(ppm)
Th
(ppm)
Internal b(AGy/a)
External a(AGy/a)
External b(AGy/a)
Cosmic
(AGy/a)External
c+ cosa
(AGy/a)
External
c+ cosb
(AGy/a)
Total dosea
(AGy/a)Total doseb
(AGy/a)Agea (ka) Ageb (ka)
1 10.5 115F 2.5 11.3 2.4 2.6 11.9 518 374 1990 35 1272 1894 4154F 268 4776F 358 27.6F 1.9 24.0F 1.9
2 9 106F 2.5 11.6 2.6 1.7 7.9 532 247 1962 45 1070 1848 3811F 234 4589F 335 27.7F 1.8 23.0F 1.8
3 4 96F 2.3 10.3 2.7 2.3 9.2 472 306 2107 90 1246 1748 4132F 260 4633F 335 23.2F 1.6 20.7F 1.6
4 0.45 55F 0.4 11.8 3.5 2.1 9.5 541 300 2627 210 1532 1771 5000F 305 5237F 356 11.0F 0.7 10.5F 0.7
5 0.7 70F 2.0 11.7 2.2 2.6 8.5 536 312 1811 190 1244 1826 3902F 236 4485F 360 18.1F1.2 15.7F 1.2
6 0.55 68F 1.8 9.6 2.1 4.2 12.5 440 481 2013 200 1565 1748 4498F 296 4682F 344 15.3F 1.1 14.7F 1.2
7 14 84F 5.1 11.3 2.9 3.1 16.0 518 479 2475 35 1610 2022 5082F 338 5494F 414 16.6F 1.5 15.4F 1.5
8 9 74F 2.2 11.8 2.7 2.1 12.0 541 382 1776 45 1224 1796 3922F 354 4494F 385 18.9F 1.8 16.5F 1.5
9 6 87F 4.4 11.5 2.5 2.9 14.0 527 431 2144 65 1448 1636 4550F 297 4738F 345 19.1F1.6 18.4F 1.6
10 0.5 71F1.7 10.5 3.0 2.0 10.5 481 312 2302 210 1459 1921 4555F 281 5017F 354 15.6F 1.0 14.2F 1.1
11 5.5 149F 5.1 10.9 3.3 2.2 15.2 500 411 2616 85 1621 2197 5147F 334 5724F 426 29.0F 2.1 26.0F 2.1
12 3.5 102F 5.6 10.3 3.1 2.0 16.0 472 414 2453 105 1606 1995 4949F 321 5334F 394 20.6F 1.8 19.1F1.8
13 0.35 66.3F 7.7 10.9 3.3 2.8 13.0 498 406 2624 220 1713 2297 5242F 332 5826F 423 12.7F 1.7 11.4F 1.6
14 0.65 59.1F 9.6 10.4 3.3 2.4 12.4 476 371 2570 190 1619 2144 5037F 319 5562F 402 11.7F 2.0 10.6F 1.9
15 1.1 68.1F 5.0 8.8 3.2 3.4 22.0 403 608 2799 180 2100 2296 6910F 402 6106F 465 11.5F 1.2 11.2F 1.2
Depth measured from present-day surface. De measured using infrared stimulated luminescence on alkali feldspars and the Single Aliquot Added Dose protocol. Grain size for all
samples: 149–177 Am. Cosmic dose estimated from burial depth. Time-averaged water contents estimated at 15F 5%.a Calculated from radioisotope contents measured in the lab.b Calculated from field measurements, attenuated for 15% water contents. These ages are used in the paper.
D.Bowmanet
al./Sedimentary
Geology163(2004)211–228
216
ary Geology 163 (2004) 211–228 217
3.2. Luminescence dating
The luminescence method dates the last exposure
of mineral grains to sunlight (Aitken, 1998), that is to
say, the age indicates the burial time of the sediment.
In case of deformed sediments, deformation occurred
when the sediment was saturated near the water–
sediment interface and the luminescence ages give the
maximum age of deformation.
This dating method uses signals that accumulate in
minerals as a result of natural ionizing radiation and
which are zeroed by exposure to sunlight. After a
resetting event the signals grow as a function of time
and environmental radiation, and therefore can be
used to estimate the time elapsed since the mineral
underwent an event of transport and burial (Aitken,
1998).
Fifteen samples for luminescence dating were
collected from the five sections, four along the south-
ern shores and one on the northern shores of Lake
Issyk-Kul (Fig. 3). In all cases, the dated beds consist
of very fine to fine sands. The samples usually bracket
deformed units in order to optimize coverage of the
deformation events. The samples were collected from
holes dug into the sections under a black tarp and
were immediately placed in black light-tight bags. All
further laboratory sample processing was carried out
under subdued orange light.
The laboratory procedures roughly follow those
described by Porat et al. (1999). Sand-size (150–
177 Am) alkali feldspars (KF) with densities less
than 2.58 g/cm3 were extracted from the sand by
heavy liquid separation, following sieving and dis-
solution of carbonates with 10% HCl. Aliquots of
f 5 mg of extracted KF were deposited on 10-mm
aluminum discs using silicon spray as an adhesive.
All measurements were carried out on a Risø DA-12
reader, equipped with an array of infrared diodes
and a 90Sr h irradiator (Bøtter-Jensen et al., 1991).
Equivalent doses were determined by the Single
Aliquot Added Dose technique (Duller, 1994),
whereby the infrared emission at 880 nm was used
for stimulation.
External c dose rates were measured in the field in
the holes dug into the sections for sample collection.
A portable Rotem P-11 g scintillator with a 2-in.
sodium iodide crystal was used, calibrated to mea-
sure cosmic rays (Porat and Halicz, 1996). The
D. Bowman et al. / Sediment
concentrations of U and Th in the sediments were
measured using inductively coupled plasma mass
spectroscopy (ICP-MS) and the K content was mea-
sured by ICP-emission spectroscopy. External a and
b dose rates were calculated from the concentrations
of the radioelements in the sediments. Internal b dose
rate was determined from the K contents of the
extracted KF. An a-value of 0.2F 0.05 was used
for a-efficiency corrections (Mejdahl, 1987; Rendell
et al., 1993).
Today, the studied sediments are dry, however, at
the time of deposition and until lake levels receded,
the sediments were water-logged. Therefore, a time-
averaged estimated water content of 15F 5% was
used in the age calculations. The ages were calculated
using the software Age developed by R. Grun. Table 1
gives all field and laboratory measurements and dose
rate calculations. Errors on individual dates were
calculated from errors on all laboratory and field
measurements, and they include uncertainties in field
data, analytical and random errors.
Gamma dose rates were obtained by two means,
(a) measurements in the field and (b) calculations
from the concentrations of the radioelements. All
values were attenuated for 15% moisture contents.
On average, the c dose rates measured in the field are
25% higher than the values calculated from the radio-
elements. Consequently, the ages calculated from the
field measurements are on average younger by
f 10% (Table 1). We chose to use the younger ages
calculated from the field measurements, as in situ cmeasurements take into account local inhomogenei-
ties in the sediment.
4. Results
4.1. Main sedimentary characteristics and facies
association of the deformation-bearing beds
The studied sections (Fig. 3) expose alternations of
well-stratified or laminated sand, mud and sandy–
pebbly beds, often showing wavy bedding, some
cross-lamination, foreset bedding and some massive
layering. The sorting is good. Mollusks and inclusions
of hydrous ferric oxides of lagoonal-lacustrine origin
have previously been reported (Markov, 1971). Such
cyclic patterns of mud and sand, often with pebbles,
Fig. 4. Washed-out circular depressions formerly occupied by
isolated balls at the top of the coastal cliff by station 15, Akterek.
Fig. 6. A giant sandstone ball with a flat upper truncation surface.
The underlying strata are undisturbed. Papers indicate sampling
sites (station 10—Choktal).
D. Bowman et al. / Sedimentary Geology 163 (2004) 211–228218
indicate dynamic facies fluctuations between the shal-
low lacustrine—beach—and fluviatile environments.
The following main characteristics were observed in
the studied sections (Fig. 3).
4.1.1. Akterek section (station 11)
A gravelly unit is overlain by fine laminated sand
with micro-ripple cross lamination, alternating with
laminated clay (samples Issyk. 5, 1633.9 m; and
Issyk. 6, 1634.1 m). The section suggests transforma-
tion from a beach/ fluviatile facies to shallow lacus-
trine conditions. Two deformed beds are present at
different elevations.
Fig. 5. Intrusive contacts between marly balls. Bedding is deformed
and preserved. The injected sand forms flame structures. Flat
bounding contacts at the top indicate postdefomational erosion prior
to deposition of the overlying beds.
4.1.2. Akterek section (station 15)
Alternations of sandy–muddy laminae (sample
Issyk. 1, 1624.5 m) coarsen upwards to sandy gran-
ules and pebbles (Issyk. 2, 1625.5 m). Above, there
is a hard muddy debris flow unit overlain by well-
stratified and laminated loose sand with well-sorted
and rounded pebbles, dipping 8j northwards (sample
Issyk. 3, 1630.2 m) and tangentially cross-bedded to
the underlying debris flow unit. The section is
capped by marly–muddy sand (sample Issyk. 4,
1634.2 m). Two deformed beds are present at differ-
ent elevations.
Fig. 7. Large-scale complex convolute bedding structure. The
features incorporate ball and pillow structures. Note the truncated
flat upper surface. Bedding is well preserved (station 15—Akterek).
Fig. 8. Detail of Fig. 7 left, by the hammer: complex convolute
bedding with recumbent and overturned folds.
D. Bowman et al. / Sedimentary Geology 163 (2004) 211–228 219
4.1.3. Irddyk section (station 18)
Sandy pebbles are overlain by alternating fine and
coarse wavy laminated sand beds (sample Issyk. 11;
Fig. 9. Deep penetration of marly ‘‘pillars’’ intruding down into pebbley co
the sand beds was completely destroyed by its liquefaction. The curved ‘‘ p
the isolated marly blocks (Fig. 10) may imply lateral flow of the sand.
1630.8 m), followed by laminated and massive and
pebbley granular sands (sample Issyk. 12; 1633.5 m).
The Irddyk section is overlain by a whitish mudstone
and includes a deformed horizon at its base.
4.1.4. Choktal section (station 10)
This comprises alternating fine-bedded sand and
silty mud (sample Issyk 15, 1612.7 m; sample Issyk
14, 1613.1 m; sample Issyk.13, 1613.4 m), including
two deformed beds.
4.1.5. Karakol section (station 17)
The base is a sandy–pebbly bed, abruptly overlain
by laminated wavy sand (sample Issyk. 7, 1622.5 m).
This is followed, across an irregular contact, by
deformed whitish mudstone, overlain abruptly by
well-bedded sand with granules and small pebbles
(sample Issyk. 8, 1628 m). The overlying micro-
rippled cross-laminated fine sand (sample Issyk. 9,
arse sand, which was injected upwards. The internal stratigraphy of
illars’’ (above, at station 18 Irdyk; Fig. 10, by the Tossor river) and
D. Bowman et al. / Sedimentary Geology 163 (2004) 211–228220
1630 m) is deformed. So there are five overlying beds,
including the topmost bed of mud and sandy pebbles
(sample Issyk. 10, 1636.4 m). The Karakol section
includes seven deformed beds.
4.2. Soft-sediment deformation features
The following main deformed features have been
observed in the sites studied around Lake Issyk-Kul:
1. Pseudo-nodules or isolated balls. These balls,
composed of sand or mud, vary from 14–18 cm long
and 4–6 cm thick to 19–50 cm long and 13–29 cm
thick. The deformations form a pear structure and are
separated by crested diapiric flame structures com-
posed of the coarse, loose and unstratified sandy host
unit. Original layering is bent around the balls and is
parallel to the basal surface. This interpenetrative type
of deformation (Allen, 1977) at the top of the coastal
cliff (Akterek station 11, Figs. 4 and 5) occurs in
Fig. 10. Deep penetration of marly ‘‘pillars’’ intruding down into pebbley c
the sand beds was completely destroyed by its liquefaction. The curved ‘‘ p
isolated marly blocks (above) may imply lateral flow of the sand.
rather regular lateral intervals in beds of uniform
thickness and often changes laterally from a ball form
to wavy anticlinal and synclinal convolute bedding.
The top of such deformed structures is commonly
sharply truncated. Their lateral extent ranges from
tens to hundreds of meters, implying little variation
in loading. No indications of ripple morphology
related to the deformation were observed.
2. Giant balls and pillows, 0.7–2.1 m long and
0.3–0.7 m thick with flame structures. These features
are marly-muddy, bounded by subhorizontal sandy–
muddy lamina and hosted in massive loose sand (Fig.
6). They differ in size from the previous category and
were observed in the Choktal section, on the northern
shore of the lake. Primary lamination remained in
most cases undestroyed; however, torn lamination was
also encountered.
3. Complex and irregular convoluted sand beds
which comprise a tabular unit, 40–60 cm thick,
oarse sand, which was injected upwards. The internal stratigraphy of
illars’’ (Fig. 9, at station 18 Irdyk; above, by the Tossor river) and the
D. Bowman et al. / Sedimentary Geology 163 (2004) 211–228 221
bounded by undisturbed and undeformed horizontal
beds, were observed at the base of the section at
section 15 (Fig. 3, altitude 1625 m). They include
(Figs. 7 and 8) vertical intrusions and large-scale
complex recumbent folds, partly in a highly disorga-
nized, irregular and chaotic pattern that might imply
some horizontal displacement. The internal lamina-
tion, although distorted, is well preserved. This unit is
traceable laterally for tens of meters. It is very
different from the regular-spaced folding of broad
synclines and pinched anticlines described by Cojan
and Thiry (1992).
4. Whitish, muddy–marly ‘‘pillars’’, 50–60 cm
high, which intrude down into sand, were observed
in the Irdyk and Akterek sections and near the Tossor
river (Figs. 9 and 10). The unit that hosts the ‘‘pillars’’
is composed of very fine to medium sand, often with
micro-ripple cross-lamination, alternating with silt and
mud. Alternatively, it is composed of massive coarse
sand with well-sorted and rounded granules or small
pebbles. The deeply intruded sand is injected upward
between the downwards-intruding ‘‘pillars’’ of marl,
some of which are in a curved position. The upper
sand–mud interface is completely destroyed. The unit
containing the ‘‘pillars’’ is bound by planar upper and
lower surfaces.
The ‘‘pillar’’ deformation is easily distinguishable
from pillows and pseudo-nodules. It is a vertically,
deeply displaced structure. It is unique being a soft
muddy–marly load on top of a coarse granular sand
which is of high initial porosity.
5. The field criteria for seismites
We use the term ‘‘seismites’’ following Seilacher
(1969) for structures formed in soft sandy sediment by
seismic shocks. Each typical field criterion (Sims,
1975; Hempton and Dewey, 1983; Anand and Jain,
1987; Obermeier, 1996a) suggested for relating de-
formation features to palaeoseismic events, though not
as compelling evidence, is discussed and related to
our observations in the Lake Issyk-Kul area.
(A) Suitable location in a seismically active area.
Lake Issyk-Kul is situated in an area where many
strong modern earthquakes have occurred (Dzhanu-
zakov and Sadykova, 1993; Abdrachmatov et al.,
2002). The epicenters in the vicinity of the Issyk-
Kul depression (Fig. 11) indicate a seismically very
active zone, mainly north of the lake. During the 101
years 1889–1990, the following strong (M>6.2) earth-
quakes were reported in the basin, some of them
ranking among the strongest ever felt in continental
areas: the Chilik 1889, MS = 8.3 earthquake (Mush-
ketov, 1899); the Kebin, 1911,MS = 8.7 (Bogdanovich
et al., 1914); the Kemin-Chue, 1938, M = 6.9 (Vil-
gelmzon, 1947); the Sary-Kamysh, 1970, M = 6.8
(Grigorenko et al., 1973); the Zahalanash-Tyup,
1978, Mb = 7.1 (Aitaliev, 1981); and the Baysoorun,
1990, MS = 6.3 (unpublished data of the Institute of
Seismology, NAS, Kyrghyzstan).
(B) Suitable sediments—loosely consolidated,
metastable sands and silts with low cohesion. Because
of these properties (Dzulynski and Smith, 1965; Mills,
1983) and following an excess of pore pressure in
water-saturated conditions and a reverse density state
sufficient to cause gravity instabilities, the sediments
may loose cohesion and liquefy. Clay-rich sediments
are generally not susceptible to liquefaction because
of their cohesiveness. Poorly sorted coarse sediments
tend to be less permeable and of greater strength. The
lacustrine sandy–muddy facies observed in the study
sites are partly porous and loosely packed, thus
meeting the basic textural requirements for plastic
load deformation.
(C) Similarity to structures formed experimentally
under conditions of earthquake-induced shaking
(Kuenen,1958; Owen, 1996) or reported elsewhere
as seismites (Seilacher, 1969; Scott and Price, 1988;
Ringrose, 1989). The deformation features revealed in
our work compare well with both soft-sediment de-
formation structures reported from the geological
record and with those demonstrated experimentally.
It is, however, noteworthy that the study area is
typified by absence of liquefaction-induced fluidiza-
tion and vented sediments (Obermeier, 1998a,b), in
form of clastic dikes, sand-filled fissures, sills and
intrusions that pinch together upwards.
(D) Preclusion of trigger by gravity flow. Seismites
should relate to areas where slope instabilities induced
by gravity control can be excluded in order to avoid
deformation induced without shaking. The lateral
continuity of the deformation structures within well-
defined beds precludes a gravity flow origin. Lami-
nated clay, silt and fine sand deposited in-between the
deformed beds suggest still-water lacustrine condi-
Fig. 11. The spatial distribution of epicenters for events M>5 recorded or known in the study area, focusing mainly on the period 1874–1990.
Data sources: Dzhanuzakov and Sadykova, 1993; the Kyrgizian Seismological Institute (personal communication); U.S. Geological Survey
Earthquake Data Base and Harvard CMT catalogue.
D. Bowman et al. / Sedimentary Geology 163 (2004) 211–228222
tions, diminishing the likelihood of occurrence of
gravity-driven density currents producing shear (Jones
and Omoto, 2000). Lack of evidence of rotational
slips, pull aparts and forward displacement of materi-
al, which is typical of slumps (Mills, 1983), decreases
the likelihood of slope control.
(E) A stratigraphically sandwiched position. The
deformed layers should be stratigraphically sand-
wiched between undeformed stratigraphic intervals.
This is shown in many cases by the undeformed,
overlying and underlying bounding strata (Figs. 5 and
6). Clear rhythmic alternation of deformed beds with
undisturbed strata may also indicate the instantaneous
nature of seismic triggering (Rossetti, 1999), implying
that deformation occurred very shortly after deposi-
tion (Jones and Omoto, 2000).
(F) Lateral continuity and regional abundance. A
wide lateral extent of the deformation structures and
their regional abundance are prerequisites for regard-
ing them as seismically triggered (Allen, 1986; Ober-
meier, 1996b). They are widely distributed along the
Issyk-Kul lake shorelines. Their abundance and ex-
tent fits the expected effect of earthquake-induced
events, although no synchroneity could be established
(Fig. 3). At each study site, the deformations are
laterally continuous for only tens to hundreds of
meters. These findings strongly corroborate Ober-
meier’s (1996a,b) conclusion about the large variations
in the abundance of liquefaction-induced features
within a local area. Nonetheless, the spatial distribu-
tion of the soft-sediment deformations is very wide
around Lake Issyk-Kul. The specific zones showing
soft-sediment deformations alternate, as elsewhere,
also according to the textural interfingering, along
the margins of the basin. Textural interfingering causes
regions of non-liquefiable deposits within the potential
area of liquefaction, complicating the spatial distribu-
tion pattern.
(G) Cyclic repetitions of structures. Cyclic repeti-
tions of structures are expected in seismic zones
following recurrent seismogenic triggering. Two to
seven vertical repetitions of discrete horizons bearing
these deformation structures were exposed in the
studied sections (Fig. 3). Such cyclicity is, however,
D. Bowman et al. / Sedimentary Geology 163 (2004) 211–228 223
by itself, not diagnostic of a seismogenic origin. It
may also indicate repetition of depositional events or
repeated wave-induced liquefaction.
6. The age of the seismites
All the samples analyzed in this study were taken
from a height range of 25 m, at altitudes between 1612
and 1637 m (Fig. 3). The time window of the 15 dates
is from 26.0F 2.1 to 10.5F 0.7 ka (Table 1), all
within late Upper Pleistocene. There is some data in
previous studies for the age control. Markov (1971)
dated mollusks by radiocarbon, at an altitude of 1633
m, 7 m below the tread of the ‘‘Nikolaevka’’ lacus-
trine terrace, a well-known marker of the region at an
altitude of 1640 m. His date, 26,340F 540 YBP, falls
within our oldest ages.
Fig. 12. Age vs. altitude of the 15 samples taken for luminescence dating. A
correlation in the Akterek—15, Akterek—11 and Irdyk—18 sections. The
line of best fit is shown excluding Irdyk and Choktal sections and sample
During a considerable part of the Holocene, in-
cluding the last 100 years, the lake level was lower
than the study area (Fig. 2). Deformations related to
that period, including the last century which was of
very intense earthquake activity (Fig. 11), must be
buried under the recent sand and beach gravel and
below the recent lake level. This conclusion is
strengthened by Ricketts et al. (2001) who collected
piston cores in the lake from which 16 AMS radio-
carbon dates were obtained up to a core depth of 40
m. The ages for the sediment ranged from 1480F 45
to 8940F 65 radiocarbon year BP.
From the point of view of the spatial correlation of
the dates, when based on altimetry, section no. 18
Irdyk stands out with its relative high, though old,
dates (Fig. 3, samples 11 and 12). Its location on the
limb of the Bir-Bash anticline (Fig. 1) may indicate
warping. Samples 13–15 from Choktal, on the north-
ge is attenuated for 15% water content. Note negative altitude–age
number and error limits of each dating are indicated. Approximate
s 4, 7 and 8.
D. Bowman et al. / Sedimentary Geology 163 (2004) 211–228224
ern side of the lake, were taken from 6 to 8 m above
the recent lake level and show the same age as sample
no. 4, almost 20 m higher, from Akterek on the
southern side of the lake. This may reflect a greater
subsidence on the down-thrusted side of the Kultor
fault compared to the down-thrusted side of the Pre-
Terskey fault (Fig. 1c).
Negative altitude–age correlation were revealed in
the Akterek-15, Akterek-11 and Irdyk-18 sections
(Fig. 12). Such correlation suggests that the ages
reflect the stratigraphical order without major inter-
ruptions by ‘‘cut and fill’’ events which often result in
formation of insets. Ricketts et al. (2001) report the
same trend for sediments below the lake level.
7. The relevance of historic seismicity
The epicentral map (Fig. 11) shows 19 earthquakes
of M>5.5 during 183 years, 1807–1990, resulting in
an average recurrence interval of 10 years with
1r = 14 years. The five strongest earthquakes (M>
6.2) during 101 years 1989–1990, which is the period
with the best data, make an average recurrence inter-
val of 25 years with 1r = 23 years. Based on the
relation between the recurrence interval and earth-
quake magnitude (Slemmons and Depolo, 1986), the
Issyk-Kul basin falls within the group of the ‘‘most
active’’ seismic rate at major plate boundaries.
The empirical relationship between maximum dis-
tance of epicenter to liquefaction site R and the
earthquake magnitude M is given by Kuribayashi
and Tatsuoka (1975) and by Vittori et al. (1991): log
R = 0.87M� 4.5. Thus, liquefaction may not occur
further than 70 km from an epicenter of an earthquake
with magnitude M = 7.0. For a distances exceeding
100 km, M = 7.5 seems to be a minimum threshold.
Such relationships were also established empirically
by Tinsley et al. (1985). Galli (2000), based on Italian
data of the period 1117–1990, showed that following
a 7 magnitude event, liquefaction may occur even in
wider magnitude–distance combinations, even be-
yond 100 km. These distances make the active epi-
central zone north of Lake Issyk-Kul (Fig. 11) with its
high magnitude (> M = 7) events, the most favorable
source for the soft-sediment deformations reported in
our study. Although our study sites were far apart,
their regional distribution was, however, not wide
enough to indicate, through the magnitude of the
deformations, the central core region.
8. Discussion
The ball-and-pillow and pseudo-nodules indicate
loading of sand above water-saturated, soft and fine
clay-rich sand and silt, and build up of pore-water
pressure, which caused the loss of bearing capacity
(Lowe, 1975; Allen, 1982). The upper sand intruded
downward the weaker beds and became detached,
kidney-shaped and often completely enclosed pseu-
do-nodules. Such deformations have been also pro-
duced experimentally by shaking (Kuenen,1958).
However, as both the seismic trigger and nonseismic
triggers, such as rapid deposition, gravity-induced
mass movements or storm wave impact, can produce
deformation (Moretti et al., 1999; Owen, 1996) these
structures cannot serve as diagnostic criterion for
supporting a seismogenic origin.
The deformation structures encountered in the
study area are typified by a relative high symmetry
and are not structureless internally. Their nearby
surfaces lack, almost entirely, ripples or other bedform
features and evidence of lateral movement and orien-
tation, suggesting lack of current and drag. The
structures are not accompanied by floating clasts or
tool marks typical of turbidites and high flow depos-
its, although recumbent folds and cross stratification,
which would be expected following current shear on
liquefied sand (Brenchley and Newall, 1977), have
been observed. The main observations suggest that the
trigger was not via the water body and that the style of
rheological behaviour of the sediment was hydro-
plastic, indicating limited local and mainly vertical
particle movements (Elliott, 1965).
Storm and breaking waves provide an attractive
alternative to seismicity as a trigger of soft-sediment
deformation. Storm wave liquefaction features due to
the cyclicity of the impact of storm waves or due to
the breaking process (Owen, 1987) are, however,
poorly documented in the literature. Dalrymple
(1979) described isolated slump bodies on the upper
stoss side of mega-ripples, separated by sharply
peaked anticlinal structures, as indicators of wave
activity. Molina et al. (1998) reported soft-sediment
deformation structures in form of casts and isolated
D. Bowman et al. / Sedimentary Geology 163 (2004) 211–228 225
water escape structures due to storm waves in marine
Miocene carbonates. Common criteria for strong
oscillatory flows at the base of storm waves, which
indicate the inner shelf and the lower shoreface, are
bedforms such as hummocky cross-stratifications and
symmetrical ripples (Walker, 1980; Brenchley, 1985;
Duke,1985; Greenwood and Sherman, 1986; Eyles
and Clark, 1986; Cheel and Leckie, 1993). These
liquefaction features and bedforms have not been
encountered in the study sites. Based on present
knowledge, we have no evidence or reason to regard
storm waves as triggers for the deformations in the
Issyk-Kul basin.
The absence of liquefaction-induced venting fea-
tures and water escape flow paths in the studied
localities, and the dominance of plastic deformation,
makes the study area fall in the worldwide category of
areas with plenty of plastic deformation, in which
vented liquefaction features could not develop (Ober-
meier, 1996b).
Lowe and LoPiccolo (1974) described pillars as
circular columns, ranging in size from 1 mm to
several meters and over 1 m across. Pillar structures
related to overloading and mass sedimentation were
reported by Ricci Lucchi (1980) and by Allen (1982).
Moretti et al. (1999) showed that fluid escape struc-
tures, centimeters in heights and similar to pillars, can
form following seismically induced liquefaction in
normal-graded beds. Pillar structures formed by flu-
idization of fine material escaping upwards were also
described by Wentworth (1966) and by Lowe (1975).
All these features are, however, very different from
the ‘‘pillars’’ we have described.
It is noteworthy that our ‘‘pillars’’ did not develop
in a reversed density system. Soft-sediment deforma-
tions in the form of ‘‘pillars’’, where a marly unit
intrudes down into coarse gravelly sand, have not
previously been reported. As marl on top of sand acts
as an impermeable layer, water must have been forced
through the sand laterally from subjacent strata.
Shaking could decrease drastically the strength of
the sandy unit which finally allowed the sinking of
the overlying marl into the sand to form the ‘‘pillars’’
observed by us. The form of ‘‘ pillars’’ indicate
detachment and sinking—and not foundering—of
the overlying stratum into the sand, strengthening
the inference for a seismic trigger (Obermeier,
1998b). As liquefaction of a sand bed requires pro-
longed cyclic stress (Seed, 1968), the magnitude of
the ‘‘pillars’’ and their wide extent may be suggestive
of a high-magnitude and/or long-duration seismic
event. We have observed also oblique ‘‘pillars’’
(Fig. 10) which may have been tilted following the
shaking.
The chaotic deformation structure we have encoun-
tered (Fig. 8) is a multilayered system which would
also require a substantial trigger to initiate break up
(Brenchley and Newall, 1977). The ‘‘giant’’ ball (Fig.
6) is an additional possible indicator of a large seismic
trigger. The absence of additional ‘‘giant’’ balls, 6–
8 m above the current lake level, seems to be related
mainly to the lack of exposures.
9. Conclusions
We cannot provide compelling evidence of seismic
triggering, such as fitting radiocarbon dates of two
different and separated, but stratigraphically correlat-
ed, features to a specific historic earthquake of the
same date (Bowman et al., 2001). Each of the single
field criteria cannot be regarded, by itself, as diagnos-
tic of a seismic origin. However, we suggest that the
accumulated field data, observed from one of the most
active seismic zones on our globe, lend credence to
the diagnosis of a seismic trigger. Rossetti (1999)
applied a similar approach regarding the seismic
origin of deformation structures in the Sao Luis Basin,
northern Brazil. We conclude that what we have
observed and described are quite possibly seismites
of late Upper Pleistocene age, 26–10 ka BP, over an
altitudinal range of 25 m, starting from 7 m above the
current lake level.
Acknowledgements
We thank Mr. P. Louppen, Ben-Gurion University
and Mrs. B. Fabian, Potsdam University for the
computer work in drawing the figures. This study
was supported by INTAS—The International Asso-
ciation for the Promotion of Cooperation with
Scientists from the Independent States of the former
Soviet Union, grant no. 96-1923. We further thank
the support of ISTC project No. KR-357. A.K. thanks
the Deutscher Akadimischer Austaushdienst (DAAD)
D. Bowman et al. / Sedimentary Geology 163 (2004) 211–228226
as well as the Alexander von Humboldt Foundation
for supporting his stay in Germany where this paper
was completed.
The manuscript benefited significantly from the
experience and constructive comments of S.F. Ober-
meier, G. Owen and the editors.
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